1. Field of the Invention
The present general inventive concept relates to an apparatus and a method for suppressing noise in a receiver, and more particularly, to an apparatus and a method for suppressing noise in a receiver that uses an automatic gain controller.
2. Description of the Related Art
In a communication system, a signal transmitted from a transmitting side is finally transmitted to a terminal of a receiving side through a wired or wireless network. In this case, the core technology to heighten the communication quality is to suppress noise that is generated during a transmitting/receiving process. The noise generated in a signal that is finally received in a receiver is roughly classified into a transmitting side noise generated in a transmitting side (terminal) and a network noise generated as the signal passes through a network.
Typically, the network noise shows the characteristic of white noise, and in order to suppress the network noise, an automatic gain controller (AGC) is generally used in the receiver.
FIG. 1 is a view illustrating the configuration for outputting an audio signal in a receiver that includes an AGC.
A signal that is encoded in and transmitted from a transmitting side is input to a receiving terminal through a network. That is, the signal transmitted through the network is input to a decoder 101 through an antenna of the receiver. The decoder 101 decodes packets of the signal encoded in the transmitting side into a pulse coded modulation (PCM) signal. An AGC 103 automatically controls the gain of the receiver so that an amplitude of an output signal is kept constant through detection of amplitude variation of the decoded PCM signal. A digital receiving volume controller (Rx Volume) 105 controls the gain value of the output signal of the AGC 103 when a receiving side user controls the volume of a terminal. A codec gain controller 107 controls the gain of a codec at a proper level with respect to the output signal of the digital receiving volume controller 105, and a gain-controlled signal is converted into an analog signal by a digital-to-analog (D/A) converter 109 to be output through an amplifier 111.
Here, the operation of the AGC 103 that suppresses the network noise by controlling a noise gate threshold value and a noise gate slope will be described with reference to FIG. 2.
FIG. 2 is a view explaining a method of suppressing a network noise using a noise gate slope and a noise gate threshold value in an AGC.
Referring to FIG. 2, a solid line indicates a noise gate slope, and a noise gate threshold value 22 means a boundary point at which the noise gate slope is differently set. The noise gate slope 29 on the left side of the noise gate threshold value 22 is set to be higher than the noise gate slope 27 on the right side of the noise gate threshold value. In accordance with the noise gate slope set as described above, the ratio of an input to an output of a signal 21 having a level lower than the noise gate threshold value 22 becomes smaller than the ratio of an input to output of a signal having a level higher than the noise gate threshold value 22. Accordingly, the signal having a level lower than the noise gate threshold value 22 has a reduced gain after it passes through the AGC, and thus the signal level is also lowered.
The AGC suppresses the network noise by using the characteristics of the noise gate slope and the noise gate threshold value. That is, the AGC can reduce the level of the network noise by setting the noise gate threshold value to a value higher than the network noise level that is measured in advance and setting the network noise level to a desired level.
However, in the process of suppressing the network noise through the AGC, a radiotelegraph phenomenon and a swing phenomenon may occur to deteriorate the call quality. The radiotelegraph phenomenon means a phenomenon of noise disappearing with generation of “whistle” sound when the noise is suppressed by the AGC after voice call is completed, and the swing phenomenon means a phenomenon of sound rolling or disappearing of a specified phoneme of sound that is smaller than the noise gate threshold value level of the AGC during the operation of the AGC. The radiotelegraph phenomenon and the swing phenomenon notably occur in a quiet environment such as midnight call in a low voice.
In general, an audio encoder of a transmitter side terminal includes a noise suppressor. The noise suppressor can almost completely suppress static noise that is generated in the transmitter side terminal, but it is difficult for the noise suppressor to completely suppress non-static noise. The static noise means noise that maintains a constant level in the whole frequency band, such as white noise, and the non-static noise means noise the level of which is changed in accordance with the frequency band. The non-static noise, which has not been suppressed, has an increased noise level as it passes through the noise suppressor. Consequently, once the noise generated in the transmitter side passes through the noise suppressor, the static noise is suppressed, but the non-static noise becomes larger.
On the other hand, as the signal on the transmitter side passes through the network, the level of the non-static noise becomes higher, and the static noise occurs even in the network. Accordingly, the noise that reaches a receiver becomes larger since the non-static noise having a heightened level and the static noise occurring in the network are added together. In order to suppress the noise of a high level as described above, the noise gate threshold value and the noise gate slope of the AGC are set to higher values, and this causes the radiotelegraph phenomenon and the swing phenomenon occur notably.
Also, once the noise gate threshold value is set to a fixed value, it is difficult to reflect the change of the noise level therein, and thus the noise is not efficiently suppressed.